Balance controls for metal detectors



p 2, 1952 D. J. TRICEBOCK ET AL 2,509,530

BALANCE CONTROLS FOR METAL DETECTORS Filed May 5, 1949 2 Sl-IEETS-SHEE'I1 INVENTORS DUNALDJ. 77225500: 3 fi4ama1m .Yaarr ATTORNEY P 2, 1952 D..1. TRICEBOCK ET AL 2,609,530

BALANCE CONTROLS FOR METAL DETECTORS Filed May 5, 1949 2 SHEETSSHEET 2INVENTOR-S Dmwpi 71214-5505.: 36

HAY END A. wax

ATTORNEY Fatented Sept. 2, 1952 BALANCE CONTROLS FOR METAL DETECTORSDonald J. Tricebock, Philadelphia, Pa., and Ray mond A. York, Syracuse,N. Y., assignors to. Radio Corporation, of America, a corporation ofDelaware Application May. 5, 1949, Serial No. 91,609

8 Claims. (01. 336779) The present invention relates to inspectionapparatus, and more particularly is an improvement in metal detectingapparatus. 3

A metal detector or inspection machine of the general type. contemplatedhas been described and claimed in a copending application, Serial No.568,045, filed December 13, 19451, by J. H, Reynolds for MetalDetectors, now Patent No. 2,513,745, dated July 4, 1950, which isassigned to the same assigneeasthepresent invention, The ge eralprinciple of operation of devices of this type, insofar as the presentinvention is .concerned, involves theestablishment of induced a1-ternating voltages which are. normally of equal amplitude and oppositephase. This may be accomplished, for example, by applying alternatingcurrents to a primary coil to establish a magnetic field andpositioning, two secondary coils in the field, the secondary coils beingconnected in opposition and so spaced that the induced voltages arenormally equal and opposite. Alternatively, two magnetic fields may beestablished by a pair of primary coils in symmetrical coupling relationto a single. secondary or to two secondary windings connected, in,opposition. The initial operating condition is normally a null balance.

In order to indicate or detect a metal particle,

the material beingtested is passed between the primary and secondarycoils by any convenient means, such as, a fabric conveyorbelt. It iswell known that the magnetic field around acoil. ex tends a considerabledistance and that any conductor or magnetic material brought into thisfield will distort it. Distortion may be due to the magneticpermeability of the metal or due to the magnetic field produced bycurrents induced in the. object if it is conductive. Consequently, theinitialbalance, which is very critical, will be upset by any objecthaving electrical properties of permeability or conductivity which is.placed in the field of the device, even at a considerable distance fromthe device.

The sensitivity of a metal detector. to small disturbances of themagnetic field depends primarily upon the perfection of the initialbalance, and the maintenance of high sensitivity under con-1 ditions ofvibration and changes in temperature normally encountered requires-ahigh degree of of-the order required for sensitive operation.Consequently, either oneorboth of these,.-scurces or 2 non-uniformitytend to introduce intothe secondary a residual voltage which cannotbe.balanced out by merely shiftingthe position of the coils. Thisresidual voltage may-be theresultant of a number of complex voltages ofphase angle. 1

Copending application Serial No. 6,047, filed FebruaryB, 1948, by Ben R.Qossick discloses how a balance for the metal detecting apparatusmay beobtained by placing metal slugs of dissimilar metals into the couplingfield. These slugs react with the field to produce two voltages'inquadrature in the detecting coilof the metal-detecton By varying theamplitude of eachot thesequadrature voltages, the voltages inducedintheIdetecting coil as a result of dissy'mmetriesof the metal detectingsystem may be ba lancedout and a complete nullobtained.

. In order that the principles disclosed in the above noted applicationof Gossick belut'ilized'for balancing a metal detector field, it isnecessary that each metal slug move over such a path that thegnadraturevoltagesinduced. in the detecting coil, due to the presence ofthe metal; should vary from a positive maximum throughizer'o-to anegative maximum (for a given metaL at ajgiven frequency, in a givenfield, there is no "change in phase angle). The mechanical arrangementfor this presents several difficulties. 'Ihe metal slugs must be formedso that there is no change in their impedance as theyare moved about inthemetal field. Facile, yet extremely precise and rigid, adjustmentisrequired because, if a metal slug i s-of a size to appreciablyafiectthebalance. of voltages, inducedfrom the. metal detector field,slight movements will appreciably unbalance these voltages. All movingparts associated with the metal slugs must be' non-metallic.

It is therefore an object of our invention to provide an improvedbalancing system for the voltages'induced in a metal detector field.

Itis a further object of our invention to provide. an improved balancingsystem for the voltages induced in a metaldetector field which isreadily adjustable yet mechanically rigid.

It is also a further object of 'ourfin'vention to provide an improvedbalancing system for the voltages inducedin a metal detector field whichis continuously variable. I

Itis astill a further object of. our invention to providain metaldetector apparatus employing metal slugs,:an improved balancing.system'for the voltages induced in a metal detector field wherein thevoltage balance is afie cted only by the metal slugs in said system.

These and otherobjects are achieved in accordance with our invention bymoving'metal spheres or cylinders throng lathe fields cf the coils whichradiate said field in a planeyat right anglesto the axisof. saidfieldandutilizing di-- electric material for positioning'and'controlling the movement of the metal elements" through said field.By the axis of the field is meant the line passing through the center ofthe coupling field and substantially at right angles to the lanes of theoscillator and detector coils.

The novel features of our invention, both as to its organization andmethod of operation, as

well as additional objects and advantages thereof, will best beunderstoodfrom' the following description of several illustrativeembodiments thereof, when read in connection with the ac: companyingdrawings in which similarly identified parts have similar functions andwherein:

Figure 1 shows, in perspective, a metal detector head having thebalancing controls of the instantinventlon,

Figure 2 isaside elevation-of the portion of the upper housing shield ofthe metal detector head containing the balancing control, with a sectionof the shield cut away for the better illustration thereof, 1

Figure 3 is a bottom plan view of the portion of the housing shieldshown in Figure 2,

Figure 4 is aside elevation of another embodiment of the invention usedin a lower housing shield of a metal detector head, and

' Figure 5 is a bottom plan view of Figure 4'.

In Figure l is shown a typical metal detector head In having an upperhousing shield 12 and a lower housing shield l4. These two housingshields are spaced apart by end supports to provide an aperture 16therebetween through which passes a conveyor belt (not shown) carryingthe articles in which metal particles sought to be detected are located.The upper housing shield l2 contains two-field coils 68, 18 (shown indotted lines) which are disposed side by side and longitudinally alongthe bottom of the upper housingshield l2. These are the coils which areexcited to set up the metal detecting field so that the axis of thefield is perpendicular to the plane of the aperturel6 in a manner thatis well'known in the art. The lower housing shield 14 contains thedetector coil H (shown in dotted lines) which is disposed longitudinallyalong the upper portion of the lower housing shield 14. Suitableopenings are provided in the upper and lower housing shields l2, 14 topermit the detector field to be established across the aperture l6. I

Two control knobs I8, 26, in accordance with the present invention, areshown at the top of the upper housing shield l2. The threaded plugs 22,24 shown protruding from the upper housing shield l2and the lowerhousing shield l4 are respectively for the purpose of coupling power tothe field coils and for coupling the detected signal from the detectorcoil to an indicator.

Referring, now, to Figure 2, it is shown therein that the two controlknobs I8, 28 are respectively connected to two shafts 26, 28. Theseshafts 26, 28 extend through the housing I2 and have pressure collars30, 32 fitted thereon. By means of two set screws 34, 36 the'pressurecollars 38, 32 are respectively fastened to the two shafts 26, 28. Aspring 38 is fastened at its center by a screw 46 and a nut 42 to thehousing shield l2. The spring 38 presses against the pressure collars30, 32 to urge them and the shafts 26, 28 to which they are fasteneddownwardly. By loosening the set screws 34, 36, the collars 30, 32 maybe moved upward or downward to obtain a desired pressure and then theset s'crew s.34, 36 are tightened again. Two pressure plates,

46 are respectively tightlygfittedzon the shafts 26, 28. A metal sphere48 and a metal cylinder 58 are each respectively held pressed against acoil support plate 52 which carries a coil cover 54 in which the fieldcoils 68 and 10 are housed. The support plate '52 holds the coils 68 and'10 flat and rigid against the bottom of the upper shield housing I2.The cylinder 50 is so positioned that its longitudinal or cylindricalaxis is maintained parallel to the plane of the coplanar coils.

The two shafts 26, 28 are counterbored to fit respectively over twobearing pins 56, 58 so that they can be rotated and yet will not movelaterally. The bearing pins 56, 56 are tightly driven into the coilsupport plate 52. The metal sphere 48 and the metal cylinder 50 are eachrespectively positioned by retainer plates 60, 62; The re.- tainerplates 60, 62 have orifices which are'large enough respectively to holdthe metal sphere 48 and cylinder 50 and yet permit them tobe rotated andthereby to revolve around the axis of their respectively associatedshafts 26, '28. The radial distance of each of the orifices from thecenters of the respective retainer platesis equal tosubstantiallyone-half the distance between the centers of the. coplanarfield. coils. The retainer plates 60, 62 are fastened .respec-. tivelyto two bearing collars 66, 64. These bearing' collars 66, 64 arerespectivelyfitted over. the shafts 26, 28 so that .eachof the shafts.26, 28 and each of the bearingcollars 66, 64 may rotate on theirrespective, commonpaxes independently.

By turning either ofthe' controls [8, 2D, the pressure by the respectivepressure plates 44, 46 on the sphere 48 and.v the cylinder '50 causesthe sphere 48 or the cylinder 50 to be rotated and thereby to revolvearound-its associated shaft axis. The path traveled by either-the sphere48'or cylinder 50, itwill be readily seen, isa circular one and is in aplane perpendicular to the field axis. All the parts of these controlsexcept the spring 38, the sphere 48. and the cylinder 50 are made of adielectric material such as Bakelite or Lucite. The sphere form and thecylinder form of the metal slugs are merely by way of illustration oftypesorforms of metal slugs which have an unchanging impedance .as theyare rotated in the coil field. They may .be used interchangeably. The:metal of, which either the sphere 48 or the cylinder 50 is made ispreferably selected in accordance with the teachings of Ben-R. Gossickin his above-identi: fied, copending applicationfor Balance Control forMetal Detection and Inspection Equipment. Accordingly, thermetal of thesphere 48 and the cylinder 50 is selected to induce, in the detectorcoil, voltages which-have a quadrature phase relation. Some examples ofvmetal which may be selected are givenin Table IIIin the above identifiedapplication of Ben R. Gossick for Balance Control for Metal Detectionand Inspection Equipmeri Some of these are magnetic stainless steelandnon-magnetic stainless steel at a-frequen'cy'of. 15.0ikc., Nichrome andcopper at 25.0 kc; onNichrome and powdered i'ronat40.0kc. I

- From Figure 3, one may see the path along which the metal sphere 48.and the metal-cylinder 56 are guided with reference to the field coilsand therefore the path they take through the field set up by the fieldcoils. In accordance with established metal detector practice, the fieldcoils 68, 10 .(shown. dotted), are wound and excited to. set upan;opposing. field withea-ch, other,

The metal slugs are located at such a. radial dis-. tance from theirrespective shafts 26 and 28 that, as they are moved through a circularpath, they will pass through the center of, the field coil .68 where thefield produced is a maximum in one direction and where, maximumquadrature voltages are induced in one phase. The slugs next pass towarda position between the coils where the resultant field becomes a minimumin view of the opposing action of the. coil fields. The quadraturevoltages are thus. gradually re-. duced to a minimum. The. slugs. nextreach the center of the field coil 'lfl-wherethe. field itproe, duces isa maximum in the opposite direction. The quadrature voltagesinduced atthat position are a maximum with the opposite phase. ,Al-. though themovementof the sphere 48' and the cylinder 50 along their respectiveorbitshas been described herein as joint, it is. to beunderstood thatthey are both independently.- adjustable.

Figures 4 and 5 show another embodiment of our invention wherein thebottom housing shield M contains the field coils B8, 10. Two controlknobs i2, 14 are respectively mounted on two shafts l5, 78, which aresupported by and rotate in bushings 58, 82. The, bushings 80, 82 arefastened to the wall of the housing shield in well known fashion. Rings84, 8B of rubber or other. suitable friction drive materials arerespectively tightly fitted over the ends. of the shafts 16, I8 andrespectively engage two pressure plates 88, 90, constituting frictiondrives therefor. On the other sides of eachof the pressureplates 88, 90are three spheres which are all of the same diameter. The spheres 92,94, 96, 98 are made of glassor Lucite and each of the remaining spheresI00, I02 is, made of a metal which will cause to be induced in thedetector coil voltages which are in quadrature with each other inaccordance with the teachings of the above-identified copendingapplication of Ben R. Gossick. Three spheres are associated with eachpressure plate .88, 9B and they are properly spaced in order that thepressureplates may, remain level under the pressure. of the frictiondrive. Each of the three spheres 92, 94, I09 and 96, 9.8, m2 is pressedtightly upwardly gainst the coil support plate 52. Two retainer platesI04, I'Eiii hold the spheres in position so that they are free to berotated and thereby to revolve around the axes of the respectiv shaftsI08, H8. The orifices in the retainer plates Ifi-l, I06 in which themetal spheres I00, I02 are respectively retained are ata distance fromthe center of said plates which is equal to the narrowest diameter ofthe -field coils. The shafts I08, IIO have enlarged square endings II2,[I4 Which support the collars H6, H8. The other ends of the shafts I03,IH) are tightly fitted or threaded into suitable openings in the coilsupport plate 52. The pressureplates 88, 90 are respectively fittedonthe collars H5, H8 which are respectively rotatably fitted on shaftsI08, H0. The two retainer plates I04, I06 are also respectively fittedon collars I20, I22 which are fitted on the. shafts Hi8, no in a mannerto be rotatable.

Figure 5 shows the position of the metal spheres H30, I62 relative tothe coils. 68, 7E]. From Figure 5, it can be seen that, as the controlknobs are rotated, the metal spheres I00, I02 will each be moved fromone side of their respectively associated coils 68, '10 to the other,and thus moved from a minimum field to a maximum field and back to aminimum field. The

induced quadrature. voltages will similarly vary from a minimum to amaximum to a minimum again. All the parts used for the controls hereindescribed are also made of Bakelite or other suitable dielectricmaterial with the exception of the metal slugs the composition of whichis selected in accordance with the teachings of the application of BenR. Gossick previously cited herein.

We have shown herein two embodiments of our invention which enable afine balance to be obtained for the voltages induced in a metal detectorfield. The apparatus shown is rigid and not subject to motion due to thevibration which apparatus in proximity with conveyor belts usuallyreceives. Although the embodiments of our inventionare shown inconnection with a. metal detector field, this should not be construed ina limiting sense, since the principles herein shown are applicable toany apparatus wherein magnetically coupled circuits are ar ranged inorder to have a balanced energy transferto another circuit. It will beapparent to one skilled. in the art that various alterations andmodifications may be made of the forms of the invention described hereinwithout departing from the spirit and scope. hereof.

What is claimed is:

1. In apparatus employing a pair of field coils for establishing anelectromagnetic field and an associated detector coil wherein voltagesinduced by said field are to be balanced, apparatus for effecting asubstantial balance of said voltages which comprises first and secondmetal means to induce phase quadrature voltages in said detector coil,said metal means being mounted for movement through said field and beingshaped to have a constant impedance during movement through said fieldin a plane substantially at right angles to the axis of said field, andmeans to move said first and second metal means independently in saidplane between the centers of said field coils whereby the amplitudes ofsaid phase quadrature voltages are varied.

2. The apparatus recited in claim 1 wherein said means to move saidfirst and second metal means independently through said magnetic fieldis made of a dielectric material.

3. In apparatus employing a pair of field coils for establishing anelectromagnetic field and an associated detector coil wherein voltagesinduced by said field are to be balanced, apparatus foreffecting asubstantial balance of said voltages which comprises first and secondmetal means to induce phase quadrature voltages in said detector coil,said metal means being shaped to have a constant impedance with rotationin said field and throughout a plane substantially at right angles tothe axis of said field, means to move said first metal means in saidplane from one side to the other of one of said field coils, and meansto move said second metal means from one side to theother of the otherof said field coils whereby the amplitudes of said phase quadraturevoltages are varied.

4. The apparatus recited in claim 3 said metal means are spherical.

5. The apparatus recited in claim 3 wherein said metal means arecylindrical and the axis of said cylinder is parallel to said plane.

6. The combination with a metal detector head having at least twocoplanar field coils positioned by a coil support plate and a detectorcoil in spaced, cooperative relation with said field coils wherein ofapparatus. for, substantially cancelling the eifects of any metaldetector field dissymmetry, said apparatus comprising first and secondmetal means for inducing phase quadrature voltages in said detectorcoil, said metal means bein shaped to have a constant impedanceuponrotation in said field and throughout a plane at right angles to theaxis: of said field, a pair of pressure plates respectively holding saidfirst and second metal means against saidcoil support plate, a pair ofshafts respectively connected to and extending through the centers ofeach of said pressure plates, saidshafts being normalto said coplanarfield. coilsand being rotatably sup; ported in a planepa'ssingbetween/and, at right angles to saidicopla'nar field coils, said shaftsalso beingspaced longitudinally. along said field coils, meansyieldingl'ylurgingsaid shafts toward said pressure plates ,whereby saidpressure plates press said first and second metal means against saidcoil support plate, and a pair of retainer plates respectivelypositioned betweenueachiof said pressure plates. and said coil' supportplate and rotatably mounted on said shaftseach for rotation in a. planeparallel to saidcoplanar field coils, said retainer plates havingapertures therein to respectively retainisaid first and second metalmeans while permitting them to rotate, and said apertures being spacedfrom'the centers of said retainer plates a distance equal substantiallyto one-half the .distance between said coplanar fieldcoil centers, therotationof said shafts causing. s'aidfirst andsecond metal means to moveeachin aplane parallelwith -theplane of said coplanar field coils andsubstantially between said fieldcoilcenters. ij

' 7. The combination with a metal detector head having at least twocoplanar field coils positioned by a coil support 'plateiandladetectorcoil in spaced cooperative relation with said field coiL-of apparatusfor substantially cancelling the efiects of the metal detector fielddis'symmetryrsaid apparatus comprising firstand secondspherical metalmeans, for inducing phase quadrature voltages in said detector coil, twopairs of dielectric spheres, respectively associated with and having thesame diameter as said first and second spherical metal means, a pair ofshafts supported by said coil support ,plate and. being respectivelypositioned on said .0011 plateon the linebise'cting each field coil, apain or, pressure plates :for respectively holding said first and secondspherical metal means and said associatedpairs:of,spheres against saidcoil support plate, saidl pressure plates being rotatably mounted onsaid'respective shafts, a pair of retainer plates respectivelypositioned between said respective pressure plates and said coil supportplates androtatably mounted on said respective shafts, said, retainerplates having apertures to respectively, retain saidfirst and secondspherical metalmeans and said respective pairs of dielectric sphereswhile permitting them to rotate, said aperturesforsaid first and secondspherical metal meansbeing spaced from the center of said'retainerplates a distance equal to one-half thei narrowest diameter of therespectivefield coils,,said pai rs of spheres being retained in theapertures in said respective retainer plates at positions to maintainsaid pressure platessubstantially parallel to said coil support plate,and apair of friction drives respectively pressing said respectivepressure plates against said first and second spherical metalv means andtheir associated dielectric spheres, the actuation of said respectivefriction drives causing said first and second spherical metal means tomove in a iplanelsubstantially parallel with the, plane. of saidcoplanar field coils and from-one side to the other of the respectivefield coils.

v 1 8, The combination with a metal detector head having at least twocoplanar field coils positioned by a -coil support plate and a detectorcoil in spaced cooperative relation with said field coil, of apparatusfor substantially cancelling the effects of the'metal detector fielddissymmetry, said apparatus comprising first and second cylindricalmetal meansrfor'inducing phase quadrature voltagesinsaid detector coil,two pairs of dielectric cylinders respectively associated with andhaving the. same diameter as said first and second cylindrical metalmeans, a pair of shafts supported bysaid coil support plate and beingrespectively-positioned on said coil plate on the line bisectingeachfield coil, a pair of pressure plates for respectively holding saidfirst and second cylindrical metal means and said associated pairs ofcylinders against and with their cylindrical axes parallel to saidcoilsupport plate, said pressure plates being rotatably mounted on saidrespective shafts, a pair of retainer plates respectively positionedbetween said respective pressure plates and said coil support plates androtatably mounted on said respective shafts, said retainer plates havingapertures to respectively retain said first and second cylindrical metalmeans and said respective pairs of dielectric cylinders while permittingthem to rotate, said apertures for said first and second cylindricalmetal means being spaced from the center of said retainer plates adistance equal to one-half the narrowest diameter of the respectivefield coils, said pairs of cylinders being retained in the apertures insaid respective retainer plates at positions to maintain said pressureplates substantially parallel to said coil support plate, and a pair offriction drives respectively pressing said respective pressure platesagainst said first and second cylindrical metal means and theirassociated dielectric cylinders, the actuation of said respectivefriction drives causing said first and second cylindrical metal means tomove in a plane substantially parallel with the plane of said coplanarfield coils and from oneside to the other of the respective field coils.

DONALD J. TRICEBOCK. RAYMOND A. YORK.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number. Name Date 1,940,769 Potter Dec. 26, 19331,954,470 Carlson Apr. 10, 1934 1,969,819 Rinia Aug. 14, 1934 2,145,742Wechsung Jan. 31, 1939 2,237,254 Broekhuysen Apr. 1, 1941 2,321,356Berman June 8, 1943 2,513,745 Reynolds July 4, 1950

